It has previously been proposed in U.S. Pat. No. 3,461,932 of Shelton et al., issued Aug. 19, 1969, to provide a process for manufacturing continuous veneer strip by joining two pieces of veneer with a joint formed between scarfed cut ends providing beveled surfaces at the edges of the veneer. This has the disadvantage that the scarfed or chamfered ends are fragile so they break easily and frequently do not mate exactly and so they must be overlapped to ensure a good joint. The joint is bonded by after applying a glue to portions of the veneer pieces adjacent the scarfed ends of the veneer and placing the overlapping pieces in a heated press to form the joint. U.S. Pat. No. 3,686,061 of Brown et al., shows a similar method for producing plywood panels using multiple sheets of veneer which are formed by joining smaller pieces of veneer with glued joints between chamfered edges which are overlapped and glued together by pressing in a traveling press having heated platens for bonding the glue to form such lap joint. However, unlike the method and apparatus of the present invention, these prior methods produce lap joints of reduced strength and of an uncertain thickness which varies from joint to joint because unlike the present invention they do not produce lap joints having square cut edges and do not employ adjustable stops to set the final spacing between platens to ensure that the joint has a pre-determined thickness which is no greater than the thickness of one sheet of veneer in the manner of the present invention. In addition, there is no indication that these prior methods heat the lap joint above the lignin softening temperature of the veneer to allow a thermoplastic flow of the wood during pressing in the manner of the present invention to produce a stronger lap joint.
It is known that wood may be changed in shape after heating it above its lignin softening temperature. The lignin softening temperature of various wood is described in the book, "Wood Chemistry, Ultrastructure, Reactions" by Deitrich Fengel and Gerd Wegener, on pages 335-338, published in 1984. It should be noted that the lignin softening temperature of wood veneer depends upon the species of wood and upon the amount of moisture in the wood. For example, as disclosed on page 337 of the above mentioned reference, periodate lignin has a softening temperature of 195.degree. C. in the dry state with virtually no water content but has a softening temperature of 90.degree. C. with a moisture content of 27.1% water. Also, the lignin softening temperature of different species of woods varies over a wide range. Thus, a range of 134.degree. C. to 193.degree. C. is given for the lignin softening temperatures of the dry state lignin wood species discussed in the above-identified reference. For example, milled wood lignin spruce in a dry state has a softening temperature of 180-185.degree. C. However, periodate lignin spruce has a dry state softening temperature of 193.degree. C. and dioxane lignin spruce has a softening temperature in the dry state of 146.degree. C. as discussed in the above reference. Thus, the softening temperature of the lignin depends not only on species and water content but also on the molecular size of the lignin molecules.
The use of adjustable stops in the press to control the spacing between the platens in the final press position ensures that a lap joint of a pre-determined thickness is obtained by the present invention and that the thickness of such lap joint is no greater than the thickness of a single piece of veneer. This is important to avoid any discontinuities in the thickness of plywood or laminated veneer lumber product formed by such a veneer sheet formed with the method and apparatus of the present invention.
It should be noted that it is previously been proposed to provide pairs of fixed stops or spacers in presses used for fusing together thermoplastic synthetic resin molded articles as shown in U.S. Pat. No. 4,443,288 of Sawada et al., issued Apr. 17, 1984. However, this patent does not relate to the formation of lap joints between pieces of wood veneer and does not disclose the use of adjustable stops to provide such lap joints with a pre-determined thickness which is not greater than the thickness of a single sheet of veneer in the manner of the present invention.
In addition, presses with heated platens have been used previously to form glue joints between pieces of veneer as discussed in the above cited U.S. Pat. No. 3,461,932 of Shelton et al., and U.S. Pat. No. 3,686,061 of Brown et al. However, there is no discussion in these patents of heating the platens above the lignin softening temperature of the veneer to form a stronger joint in the manner of the present invention. Instead, the platens are heated merely to cause the glue to bond the pieces of veneer together which would be at a much lower temperature than the lignin softening temperature in most cases.
As shown in U.S. Pat. No. 4,507,162 of Iwamoto issued Mar. 26, 1985 and U.S. Pat. No. 4,725,325 of Hasegawa issued Feb. 16, 1988, it is old to make a laminated wood product formed of a plurality of veneer sheets which are simultaneously glued together to form the product. However unlike the present invention, the outer sheets of the laminated product are not separately formed by joining two veneer sheets together with a p joint formed by overlapping square cut edges of adjacent veneer sheets and then used later to bond the outer sheets with inner sheets. Instead, the entire product is formed simultaneously in these patents.